5.5. Alternative Design Solutions for the Array

5.5.1. Market Context

  1. The scale and urgency of the need for offshore wind as described in the Planning and Need Statement necessitates solutions that maximise the feasible installable capacity at each available offshore site.
  2. Historically, offshore wind development has been developed in easier to reach locations that are in shallower, nearer-shore locations. However, in order to optimise Scotland’s full offshore wind potential, projects must increasingly be sited in deeper water locations. If only the most optimum and least-constrained shallower water sites are used, Scotland’s deeper water resource will remain unutilised.
  3. Additionally, if sites identified as having significant wind generating potential are used sub-optimally, a higher number of projects will be required, typically in ever more challenging and constrained locations, to hit renewable generation targets.
  4. The consideration of alternative solutions must, as identified above, be undertaken on the basis of what is legally, technically and commercially feasible. There is therefore a degree of judgment involved in exercising these decisions, drawing on experience, available information, industry knowledge and analysis of future market trends.
  5. It is relevant then for the Scottish Ministers to give weight to the experience and expertise of the Applicant.  The Applicant holds vast experience in the renewables sector and an ever-growing portfolio in the offshore wind sector, as described in detail in section 1.3 of the Introduction (volume 1, chapter 1).
  6. The nature and viable scale of a floating offshore wind farm has to be considered in the context of the individual site, including factors such as water depth, as well as grid connection availability within a highly competitive grid framework. A full list of the factors that have influenced the Array Project Design Envelope (PDE) in addition to environmental constraints includes:
  • Grid connection availability and capacity;
  • Viable generation capacity (GW size) to optimise the secured grid connection capacity;
  • Commercial requirements prescribed by funding mechanisms (such as CfD);
  • Technology availability, cost and reliability;
  • Health and safety considerations;
  • Supply chain capacity and availability;
  • Project execution schedule.
  1. The Applicant has undertaken a detailed appraisal of all elements of the PDE to provide for all feasible mitigation to be deployed. The Array has developed and adopted a number of commitments to eliminate and reduce adverse effects as part of the pre-application phase (as far as practicable). These are outlined in the Commitments Register (volume 3, appendix 6.3).
  2. The final PDE for the Array is informed by expert judgment combined with market leading expertise of the realities and challenges of delivering viable projects in the marine environment. The Scottish Ministers can have confidence that the Applicant has considered all feasible options in forming the PDE to avoid or reduce harm to protected sites whilst delivering a viable project.

5.5.2. Scope of Consideration of Alternative Design

  1. The scope for consideration of feasible alternative solutions has been considered throughout the development process for the Array and has been a fundamental driver for design and decision-making. Details of refinements to the PDE are set out in Site Selection and Consideration of Alternatives (volume 1, chapter 4)
  2. The AEOI identified in the RIAA would arise from collision and/or displacement risk related to the operation of the wind turbines, and so the primary project design parameters which may influence these impact pathways during operation are considered to be:
  • Array location;
  • Array size and turbine number; and
  • Individual turbine parameters, including height of turbine blades above sea surface
  1. It should be noted that the specific layout of a project’s turbines within a site boundary (i.e. the location of each turbine) does not materially affect ornithological impacts resulting from seabird collision risk or displacement as this is not a parameter that is incorporated within displacement and collision risk models.
  2. The justification for the Array location (and the absence of feasible alternative locations) has been set out in Section 5.4.6 of this Report. Accordingly the further potential alternative design solutions considered during this stage of the Derogation Case are:
  • Array size and the number of turbines, which is aimed at optimising the generation potential of the Array;
  • The blade tip height above sea surface, which has been subject to detailed consideration of technical and commercial constraints of turbines and floating foundations whilst balancing impacts on ornithological receptors.
  1. Consultation has been a key part of the design refinement for the Array and has been undertaken through informal and formal consultation activities as detailed in Stakeholder Engagement and Consultation (volume 1, chapter 5). The Applicant has also considered data analysis, constraints mapping and other information sources to help identify environmental constraints.

                        Array size and number of turbines

  1. The explanation and justification of the Array PO Area is set out in Section 5.4.6 of this report. Subsequent to the award of the Option to Lease Agreement to the Applicant of the E1 East PO Area, the Area has been subject to geological assessment, as well as site condition surveys and other environmental surveys, that have informed the development of the Array size and number of turbines.
  2. Wind climate conditions were assessed and modelled, giving insight into the wind resource at the E1 East PO Area, and allowing the Applicant to make robust informed decisions regarding site layout, design and optimal array alignment reflective of wind directional distribution and wind turbine stability. In addition, FLiDAR data provided an understanding of the wind climate across the full rotor diameter, which provided an appreciation of site characteristics and determination of wind turbine suitability.
  3. The Applicant’s desktop geological assessment of the E1 East Plan Option area identified an average water depth of 73 m Lowest Astronomical Tide (LAT). These depths were considered to be suitable for the siting of floating turbines, with discrete locations shallow enough to accommodate offshore substation platforms (OSPs). Water depth was a key driver for the selection of floating technology as the preferred solution for the wind turbine generators. Due to the limited areas of shallow depth that could accommodate fixed foundations the Applicant aimed to deploy a single floating foundation solution across the entire site. A floating solution across the site supports streamlined engineering and technical approaches, is more efficient from a cost perspective and helps to ensure eligibility for a single and more competitive floating CfD bid.
  4. Site-specific geophysical surveys conducted across the Array provided greater detail on the site bathymetry and seabed conditions which indicated that there was a greater area of favourable ground conditions for turbine installation that could support an increased capacity to maximise the lease area potential. This has resulted in an increase from 145 wind turbines (originally put forward in the ScotWind bid) to up to 265 (being put forward within the EIA).
  5. If an amended array size and reduced number of turbines is considered, the following observations are made:
  6. It would not be possible to ensure that the current project capacity could be achieved if the E1 East PO area (the current Array site boundary) was reduced.  Reduction in spacing between turbines can result in wake effects that can impact energy yield. Minimum spacing for turbines is a requirement of Marine Guidance Note 654 (MGN654) (MCA, 2021) which provides guidance to accommodate safe and effective search and rescue operations in the event of an emergency.
  7. A smaller site boundary for the Array and reduced number of turbines would not support the Array project objectives: 1 (supporting Scottish and UK Government decarbonisation and climate change targets); 4 (deploying floating wind technology at scale to generate renewable power at greater depths); 5 (leading a step change by deploying floating technology at scale); or 6 (facilitating socio-economic development within the floating wind sector). A reduction in turbine numbers would only be feasible if individual turbine capacity is increased, and this is not considered technically viable for the floating offshore wind market.  Therefore a reduction in turbine number would represent a failure to achieve 3.6 GW installed capacity and is considered an inefficient use of the E1 East PO Area, a failure to maximise energy yield and a failure to maximise economies of scale.
  8. Furthermore, the Array must compete for a CfD in a competitive tender, without which it may not attract finance. An unviable project would not contribute to the mitigation of the climate emergency and would not help to address the security of energy supply risks. A failure to maximise generation and export capacities is anathema to the core project objectives of decarbonisation, supporting floating supply chain, and ultimately making a substantial contribution to meeting climate policy goals via renewable energy generation.
  9. For these reasons, alternative array size and turbine reductions are not feasible alternative solutions to the Array.

                        Height of turbine blades above sea surface

  1. Within the Array EIA Scoping Report (Ossian OWFL, 2023), the Applicant committed to identifying an appropriate air gap to reduce and mitigate collision mortality of seabird species that may forage in the vicinity of the Array. Development of floating wind turbine foundations is still in relative infancy when compared to fixed bottom sites, therefore, there remains uncertainty over the limitations of floating foundations to support larger wind turbine towers. Due to the horizontal and vertical movements resulting from the pitch of floating foundations, increases in minimum blade tip clearance can result in increased stress on the tower, and greater wear on the turbine generator within the nacelle. The full implications of this on offshore turbines is not fully understood due to reliance on experience from smaller demonstrator sites with limited deployment periods. However, increased turbine heights could result in reduced operational lifespans of wind turbine generators and require additional design elements to stiffen the tower and floating foundation and associated mooring and anchors therefore impacting the technical and commercial feasibility of Ossian.
  2. A precautionary comparison of collision mortality, based on preliminary collision risk modelling, was undertaken to understand the point of diminishing returns. The minimum blade tip clearance has been set at 36 m to balance the reduction in collision mortality whilst remaining cognisant of the potential limitations of floating foundations and the uncertainty around feasibility of deployment of wind turbines with larger towers. At 36 m (above LAT) the greatest reduction in collision mortality is achieved with the smallest increase in minimum blade clearance.
  3. For these reasons, an increased minimum turbine blade height is not technically feasible and therefore is not an alternative solution to the Array.
Table 5.7:
Performance of Alternative Design Options Against Array Objectives

Table 5.7: Performance of Alternative Design Options Against Array Objectives